1. Field of the Invention
The present invention relates to an information processing apparatus, a control method for the same, and a computer-readable storage medium, and in particular to technology for detecting a head region in an image of a person captured by an imaging apparatus (camera).
2. Description of the Related Art
In recent years, attention has been placed on a function for automatically controlling camera focus and exposure, camera panning, tilting, and zooming, and the like by specifying and tracking the position of a designated person in an image or video captured by an imaging apparatus such as a camera.
In order to specify a person's position, generally the person's face pattern is detected, and the movement thereof is tracked. Examples of such technology for detecting a face in an image include the various techniques disclosed in M. H. Yang, D. J. Kriegman, N. Ahuja, “Detecting Faces in Images: A Survey”, IEEE Trans. on PAMI, vol. 24, no. 1, pp. 34-58, January, 2002. In particular, due to its high detection rate and execution speed, the AdaBoost base technique by Viola et al. is widely used in face-detection technology (see P. Viola, M. Jones, “Robust Real-time Object Detection”, Proc. of IEEE Workshop SCTV, July, 2001).
In order to specify a person's position, it is not sufficient to merely detect and track the person's face pattern. There are cases where a face pattern cannot be detected due to the person's face turning to the side, turning around, or the like. Detecting and tracking a head region instead of a face is an effective alternative technique.
In order to detect a head region, a technique of detecting an elliptical shape with use of Hough transformation has been known for a long time (see R. O Duda, P. E. Hart, “Use of the Hough Transformation to Detect Lines and Curves in Pictures”, Comm. ACM, vol. 15, pp. 11-15, January, 1972). Also, in recent years, Stan Birchfield and others have been researching the detection of the elliptical shape of a head with use of gradient information regarding the circumference of an ellipse and color histogram matching in the interior of an ellipse (see Stan Birchfield, “Elliptical Head Tracking Using Intensity Gradients and Color Histograms”, Proc. IEEE International Conference On Computer Vision and Pattern Recognition (CVPR '98), Santa Barbara, Calif., pp. 232-237, June, 1998).
Although images and videos in which a person appears are individually different, generally the edge at the top of the head is relatively stable and is a good feature indicating the head. However, many upper body edges exist below a person's head, and there are often horizontal edges and vertical edges in the background region. These edges may be detected as head ellipses when Hough transformation or the technique developed by Stan Birchfield and others is used to detect a head ellipse, and there are cases where the edge at the top of the head, which is the feature indicating the head, cannot be properly detected. Also, depending on the combination of a background edge and a head edge, they may be detected as a large elliptical arc, and there are cases where the edge at the top of the head cannot be properly detected.